1. Field of Invention
This invention related generally to electroplating, and more particularly to an anodic assembly constituted by a basket of electrically conductive material containing a replenishable pile of solid pieces of the metal to be plated.
2. Prior Art
Electroplating is the process of coating an article with a thin layer of a metal through electrolytic deposition. The article to be plated is immersed in an electrolytic bath and serves as the cathode in the electrolytic system. Also immersed in the bath is an anode composed of the plating metal. A low voltage is applied between the cathode and the anode, causing a current to pass through the electrolytic solution which electrolyzes and plates the cathodic article with the anodic metal to the desired thickness. In this way, articles may be plated with silver, copper, cadmium, nickel, and a variety of other metals.
The present invention is primarily concerned with anode structures for electroplating pure tin, various tin-lead alloys, copper and other metals for which the appropriate electrolytic bath is fluoborate acid (HBF.sub.4) Electroplating in a fluoborate bath is now used on a large scale in the manufacture of printed circuit boards in which the metal plated on an insulated substrate forms the conductive lines of the electrical circuit.
A professional electroplater in any given situation is concerned not only with carrying out this operation efficiently and at the lowest possible cost, but also with producing platings which are uniform in thickness and of good quality. In the context of printed circuit boards, uniform platings of good quality are vital to the proper operation of the circuit. Three main factors come into play in determining the uniformity and quality of plating.
The first factor is constant anode area. In the conventional bar-type anode used in electroplating, as the anode is consumed in the course of electroplating, the bar takes on the form of a spear or an irregular sponge-like mass whose over-all surface is much reduced with respect to the original anode. While at the outset of the plating process one may attain a fairly constant level of metal ions in solution, with the inevitable shrinkage of the anode in the course of dissolution, it is no longer possible to maintain optimum plating conditions.
As conventional anodes of the bar type corrode, they not only assume an irregular shape but they lose weight, and when the weight is reduced to about 10% to 40% of its original value, the anodes must be replaced. The plating operation must therefore be shut down to permit the used-up anodes to be pulled out and replaced with new anode bars. This is a costly and time-consuming operation, and it gives rise to expensive scrap losses.
The second factor is constant current flow. In order to realize platings of uniform quality and thickness, the intensity of current flowing through the system must remain unchanged in the course of operation. But with bar anodes, the effective resistance of the anode varies as the anode corrodes, and the level of current flow is therefore uneven.
The third factor is contamination. In order to produce smooth, clean platings of good quality it is essential that the electrolytic bath be free of foreign ions. But should the anode structure include contaminants which electrolytically dissolve into the bath, the resultant platings will be of poor quality.
To obviate the drawbacks incident to the use of bar anodes, it has been known to provide anode assemblies wherein the anodic metal in piece form is contained in a plastic basket or a plastic-coated metallic basket. But since such baskets are electrically non-conductive, it becomes necessary, in order to electrify the structure, to place conductive strips or rod electrodes within the insulated basket to make contact with the anodic pieces therein.
Such plastic basket arrangements have several serious disadvantages. The strips or electrodes used to conduct the current to the anodic pieces are themselves subject to chemical attack by the electrolyte and hence have to be replaced periodically. Moreover, the strips or electrodes have a relatively small surface area and afford a poor and varying electrical contact with the anodic pieces. In the case of plastic-coated baskets, should the plastic skin be damaged the underlying metal is exposed and subject to attack by the electrolyte, as a result of which the basket is gradually destroyed and the electrolyte contaminated.
It has also been known to provide anode baskets formed of an electrically conductive metal such as steel. The difficulty with such arrangements is that these basket metals are chemically reactive with the acidic electrolyte normally used in electroplating, hence gas is generated and foreign ions are dissolved in the electrolyte which degrade the quality of the plating. Furthermore, such baskets, because they corrode, have a limited life and require replacement.
In my prior U.S. Pat. No. 3,300,396, whose entire disclosure is incorporated herein by reference, there is disclosed an anodic assembly for electroplating nickel, zinc and other metals. The assembly is constituted by a basket formed of titanium having small, closely-spaced openings therein, the basket being filled with a replenishable pile of anodic metal pieces, such as zinc balls.
The anodic assembly as well as the article to be plated which functions as a cathode are immersed in an acidic electrolyte, whereby when a voltage is applied between the anodic assembly and the cathodic article, the resultant current flow causes electrolytic dissolution of the anodic pieces, but not of the titanium basket. This anodic assembly is adapted to maintain the effective surface area of the anodic plating metal and the current distribution characteristics substantially constant. The metal ions in solution are maintained at a substantially uniform level to provide uniform platings of good quality.
As noted in my prior patent, titanium is an inherently active metal that is normally subject to corrosion in acids of the type ordinarily used in zinc, nickel and copper plating. However, when used in an electroplating system for these metals, the corrosion resistance of titanium is enhanced by an oxide film formed on its surface in the course of plating. This thin and adherent oxide film is chemically resistant to most acidic electrolytes. However, it has been found that in a fluoborate acid electrolyte, titanium is not rendered corrosion resistant and therefore is not suitable as a basket material for containing the anodic pieces.